Airtel — excess/wrong billing

Address:

600092

Before going to canada went to airtel store and informed them that i was planning to use whatsapp only using local wifi. I was under the impression i need international roaming to activate whatsapp. The store did not tell me that international roaming is not necessary for using whatspp. And they activated international roaming with zero rental.
In canada did not make or receive on call or sms. But to my surprise was billed 27, 000 for the month of june 19. I have not knowingly used any of the airtel services, was using only whatsapp. Got hooked to the local service mobile data automatically without my asking. Had paid the normal charges before the due date, airtel now threatens to disrupt the service if the amount is not paid.